Cytokinesis, the physical division of one cell into two daughter cells, is the final stage of the cell reproductive cycle and the least well understood. Correctly timing the process of cytokinesis so that it occurs only after chromosome replication and segregation is necessary to prevent catastrophic genomic instability and accordingly, cytokinesis is strictly regulated in concert with other events of the cell cycle. We have made significant progress in identifying and characterizing proteins essential for cytokinesis using a leading model organism for cytokinesis studies, the fission yeast Schizosaccharomyces pombe. We now propose to gain a better understanding of how these myriad proteins work together and under cell cycle control to mediate successful cell division. We will focus on two essential, conserved proteins that are necessary for assembling an actomyosin-based contractile ring that is used to pinch cells in two - 1) Cdc15 and 2) Cdc12. Cdc15 is a scaffold of the contractile apparatus and it links the actomyosin contractile ring to the plasma membrane through its F-BAR domain. We found that bulk dephosphorylation of Cdc15 at mitotic entry induces a conformational switch in the protein that allows it to oligomerize, bind the membrane and act as a stable membrane-anchored scaffold for cytokinetic ring assembly. We will now determine how Cdc15 interacts with membranes, how it oligomerizes to form a scaffold structures, and how it might influence the membrane at the division site guided by a newly obtained X-ray crystal structure of its F-BAR domain. We will study how Cdc15 is coordinated with a second F-BAR protein to achieve the proper coupling between the membrane and actin polymerization during cytokinesis. Cdc12 is the formin that nucleates the F-actin of the contractile ring. How formins are deployed for cytokinesis is incompletely understood also, and we will test the hypothesis that transient mitotic phosphorylation of the cytokinetic formin Cdc12 relieves its autoinhibition so that it functions precisely at the correct cell cycle stage. We will complement these focused mechanistic studies with proteomic and large-scale genetic screens designed to establish a functional interaction network of cytokinesis components. Although some of the details will vary between organisms, these studies will have a major impact for understanding how cytokinesis is orchestrated in multiple species including humans.